1. Field of the Invention
The present invention relates to a method and apparatus for measuring fluorescence lifetime, which can efficiently determine the lifetime and fluorescence decay curve of the fluorescence generated from a fluorescent material irradiated with excitation light.
2. Related Background Art
Conventionally known as a method of measuring the lifetime of a fluorescent material contained in a sample is "time-correlated single-photon counting." The conventional time-correlated single-photon counting is introduced, for example, by David J. S. Birch and Robert E. Imhof, in: Topics in Fluorescence Spectroscopy, Vol. 1: Techniques (J. R. Lakowicz, ed.), pp. 1-95, Plenum Press, New York (1991). In time-correlated single-photon counting, the sample is irradiated with a pulse of light having such a weak excitation power that the average number of photons in the fluorescence detected per pulse of the excitation pulse of light is significantly smaller than 1 (e.g., 0.01), and then the fluorescence photon detection time, extending from when the sample is irradiated with the excitation pulse of light until the first fluorescence photon is detected is measured. While the irradiation with an excitation pulse of light is repeated, the fluorescence photon detection time is measured for each occurrence of an excitation pulse light, thereby producing a histogram concerning the fluorescence photon detection time. Then, the fluorescence lifetime is determined according to the obtained histogram. Here, a photomultiplier is usually used for detecting fluorescence photons.